In the past, a vehicle-mounted radar apparatus derives a position of a target with respect to a vehicle (radar apparatus) by transmitting a transmission wave from a transmitter antenna and receiving a reflection wave from the target by a receiver antenna. The detailed process thereof is as follows. A signal processing unit of the radar apparatus generates a beat signal by mixing a transmitting signal corresponding to the transmission wave of which a frequency is changed in a given period, and a receiving signal corresponding to the reflection wave. That is, the signal processing unit generates the beat signal based on a frequency difference (beat frequency) between the transmitting signal and the receiving signal in each period including an up period in which the frequency ascends in a predetermined period, and a down period in which the frequency descends in a predetermined period.
Then, the signal processing unit generates a signal (hereinafter referred to as a transformed signal) for every frequency by processing the beat signal through FFT (Fast Fourier Transform). Among the transformed signals, a signal exceeding a threshold of a predetermined signal level is extracted as a peak signal. The signal processing unit derives paired data by pairing the peak signal of the up period and the peak signal of the down period based on a given condition.
The signal processing unit derives a distance (hereinafter referred to a vertical distance) from the vehicle to the target, or a distance (hereinafter referred to as a horizontal distance) of the target with respect to a vehicle running in a direction substantially perpendicular to a traveling direction of the vehicle. Further, the signal processing unit derives a relative speed of the target with respect to the vehicle. In this way, the signal processing unit derives the position of the target including the vertical and horizontal distances of the target, and the relative speed, based on the pair data which is subjected to the pairing process.
The signal processing unit outputs information (hereinafter referred to as target information) about the target, including the position of the target and the relative speed of the target, to a vehicle control device for controlling a behavior of the vehicle. As a result, the vehicle control device performs necessary vehicle control according to the target information outputted from the radar apparatus.
If a plurality of targets exist around the vehicle and the receiver antenna simultaneously receives reflection waves from the plurality of targets, a plurality of peak signals are extracted from a frequency axis in the up and down periods after the FFT processing is performed. Further, as well as the peak signals corresponding to the targets, a signal resulted from a noise also appears individually in the vicinity of a frequency of the plurality of peak signals. If the signal resulted from the noise exceeds the threshold, it is extracted as a peak signal. In the case where a plurality of peak signals exist on the frequency axis of the up and down periods, there is a possibility which satisfies a desired condition where paired data (hereinafter referred to as mis-paired data) obtained by pairing a combination of peak signals not in a correct manner but in a wrong manner is regarded as paired data (hereinafter referred to as normal-paired data) obtained by pairing a combination of peak signals in the correct manner. If the mis-paired data is derived, wrong target information different from actual target information is output to the vehicle control device, and the vehicle control device controls the vehicle based on the wrong object information, so that there is a possibility that necessary vehicle control cannot be carried out, or unnecessary vehicle control can be carried out. For this reason, in the case of paring the peak signals of each period, it is necessary to perform the pairing the combination in the correct manner.
As compared with the above-described method, Patent Document 1 discloses a technology in which a signal processing unit of a radar apparatus derives a Mahalanobis distance based on three parameters corresponding to each peak signal of up and down periods, and selects normal-paired data among the plurality of paired data, based on the Mahalanobis distance.
Specifically, three parameters used when the signal processing unit derives the Mahalanobis distance include a power difference (hereinafter referred to as a peak power difference) of the peak signals between the up period and the down period, an angular difference (hereinafter referred to as an angular difference) based on phase information of the peak signals between the up period and the down period, and an angular spectrum difference (hereinafter referred to as an angular power difference) based on phase information of the peak signals between the up period and the down period. And, the signal processing unit calculates a coordinate position in a three-dimensional space determined from values of these parameters in the pair data, and the Mahalanobis distance with regard to the coordinate position in the three-dimensional space set based on average parameter of the normal-paired data calculated in advance. As a result, the signal processing unit selects the paired data, of which the Mahalanobis distance is minimized, among the plurality of paired data as the normal-paired data.
Patent Document 1: Japanese Patent Application Publication No. 2012-103118A
However, in the case where the signal processing unit calculates the Mahalanobis distance, based on three parameters, to select the normal-paired data, as described above, there is a possibility that accuracy of selecting the normal-paired data is not sufficient. As one method of improving it, it is considered a method of increasing the number of parameters (e.g., three parameters are increased by four or more) to derive the Mahalanobis distance. However, since the Mahalanobis distance is calculated by matrix computation, there will be an increase in a processing load generated when the signal processing unit selects the normal-paired data if the number of the parameters is simply increased. And, since a time required for that the vehicle control device controls the vehicle based on the target information, there is a possibility of requiring a time over a required amount until the vehicle control device controls the vehicle based on the target information.